NOTA BIOLOGY SEM1 2021_2022

Differences between xylem and phloem

XYLEM PHLOEM

It consists of mainly dead elements It consists of living tissue

Lignified cell wall Do not have lignified cell walls

Have pits Do not have pits. They have plasmodesmata

instead.

Xylem conducts water & minerals from roots to Phloem translocate photosynthesis product,

aerial parts of the plant organic substances for example :sucrose from

leaves to storage organs & growing parts of plant

body

2.4 CELL TRANSPORT

a) Overview the various transport mechanisms across the membrane (CLO1)
b) Explain the various transport mechanisms across the membrane (CLO3)

i) Passive transport: Simple diffusion, facilitated diffusion and osmosis
ii) Active transport: Sodium-potassium pump and Bulk transport

(endocytosis and exocytosis)

a) Overview the various transport mechanisms across the membrane (CLO1)

b) Explain the various transport mechanisms across the membrane (CLO3)

1) Passive Transport

i. Movement of molecule, ion & atoms across cell membrane from a region of higher
concentration of substances to a region of lower concentration of substances.

ii. The process does not require energy.
iii. Occur in both living & non-living organisms.

Diffusion Facilitated Diffusion Osmosis
Definition: Definition: Definition:
- Movement of the - The passive transport of ions Osmosis is the movement of
substances (molecule, ion & or molecules by a specific water molecules from a
atoms) from a region of carrier protein, across a region of higher water
higher concentration to a biological membrane from a potential to a region of lower
region of lower region of higher concentration water potential through a
concentration. of substance to a region of selectively permeable
lower concentration of membrane.
- Does not require transport substance.
protein & energy.
- Does not require energy.

- Two major groups of integral
membrane proteins are
involved in facilitated diffusion:
Carrier Proteins
Channel proteins

Osmosis - Concept of Water Potential

i. Water potential (Ψ) is the term given to the tendency for water molecules to enter or
leave the solution by osmosis.

ii. Pure water therefore has the highest water potential.
iii. If two systems containing water are in contact, the random movements of water molecules

occur.
iv. Result in the net movement of water molecules from the system with the higher water

potential to the system with lower water potential until the concentration of water molecules
in both system is EQUAL

Water potential (Ψ)

i) Ψ predict / measure the DIRECTION of water in an osmotic system.
ii) Water molecule move:

- From hypotonic solution to hypertonic solution.
- From [water] to [water]
iii) Direction of water movement depends on: Solute potential & Pressure potential
iv) Water diffuses from region of high water potential (less negative or zero value) to a region of
lower water potential (more negative value).
v) Ψs (or osmotic potential) value is always negative. The greater solute concentration, the more
negative its Ψs value. Ψs predict/ measure the change in Ψ with the presence of solute molecules.

Solute potential (ΨS)

i) The effect of dissolving solute molecules
in pure water is to reduce the
concentration of free water molecules,
resulting in a lower water potential (have
less kinetic energy).
ii) All solutions therefore have lower water
potential than pure water.
iii) The amount of this lowering is known as
the solute potential (solute potential was
previously referred to as osmotic pressure
or osmotic potential).

Pressure potential (ΨP)

i. If pressure greater than atmospheric pressure is applied to pure water or to a
solution inside a partially permeable bag – water potential increases.

ii. Hydrostatic pressure to which water is subjected is called pressure potential or refer
to the component of water potential due to the hydrostatic pressure that is exerted
on water in a cell

Hypertonic Isotonic Hypotonic
Plasmolysed Flaccid Turgid

Plant cell in hypertonic, isotonic & hypotonic solution

Hypertonic Isotonic Hypotonic
Crenate Normal Lysed

Animal cell in hypertonic, isotonic & hypotonic solution

2) Active Transport

i) Na+ K+ pump (sodium - potassium pump)
ii) Bulk Transport - Endocytosis & Exocytosis
iii) Require energy/ATP.
iv) Molecule moves from a region of lower concentration to a region of higher

concentration gradient.
v) Movement of small and/ or large polar molecule.
vi) Involves carrier proteins in plasma membrane.
vii) Occur in living system/ organism.

A) Sodium – Potassium pump

Three (3) intracellular Na+ ions bind to the specific carrier protein (sodium -
potassium pump).

Step 1 The affinity for Na+ is high when the protein has this shape

Step 2 Binding of 3 Na+ stimulates phosphorylation (addition of phosphate group).
ATP is hydrolysed to become ADP + energy + Pi (phosphate group).
Phosphate group bind to the carrier protein on the inside of the membrane .

Step 3 The phosphorylation changes the transport protein’s shape.
Thus, 3 Na+ ions is expelled to the outside of the membrane.

The new shape has HIGH AFFINITY FOR TWO POTASSIUM, 2 K+ ions,
Which bind from the outside of the membrane and triggers releasing of the

Step 4 phosphate group.

Loss of the phosphate group restores the protein’s original conformation.

Step 5

2 K+ is released, affinity for Na+ is high again & the cycle repeats.

Step 6

B) Bulk Transport
i) Certain particles are either too large to pass through the small pores in the membrane or too
hydrophilic to diffuse the phospholipids bilayer of the plasma membrane.
ii) These materials are transported into or out of cells by bulk transport.
iii) Bulk transport is defined as the transport of material into or out of a cell by enclosing it within
a vacuole or vesicle.

Vesicle Vacoule
- A small, usually fluid-filled, membrane- - A space within the cytoplasm of a living cell
bound sac within the cytoplasm of a living cell. that is filled with air, water or other liquid,
- Vesicle from part of the Golgi apparatus. cell sap or food particles.
- In plant there is large vacuole bounded a
single-layered membrane (tonoplast).
- Animal cells usually have several small
vacuoles.

Endocytosis Exocytosis
Cellular uptake of macromolecules & Meaning moving substances out of a cell.
particulate substances by localized regions of
the plasma membrane that surround the A vesicle containing the substance to be
substance & pinch off to form an intracellular removed from the cell is moved towards the cell
vesicle. surface membrane with the help of
A process where the cell can transport large microtubules, using energy from ATP.
quantities of material (solids or liquids) into the
cell.

- During endocytosis, infolding or invagination The vesicle (secretory vesicle) fuses with the

of the plasma membrane around the particle. cell surface membrane releasing its contents to

- Leads to the formation of vesicles or vacuole. the outside.

2 Types of Endocytosis: Phagocytosis & Pinocytosis

Phagocytosis Pinocytosis

Material taken in is large particulate (food) / Material taken in is liquids/ fluids/ small

bacteria /large fragments / solid substance. particulate/ solutes.

Involves formation of vacuoles// Involves formation of (small) vesicles //
pseudopodium// invagination of membrane grooves.

Material are digested & absorbed into Material // dissolved substances / fluids
cytoplasm absorbed directly into cytoplasm & not involve
Involve lysosome. lysosomes.

- Plasma membrane projects itself outward to - The small particulate of liquid is attracted to
form pseudopods. the surface of plasma membrane.
- Pseudopods surround the large fragments - The membrane folds inward (invaginate) to
(food) outside the cell & bring it into the cell form pinocytic vesicle and breaks off from the
forming a membrane sac around the particle. plasma membrane.
- The useful end-product are absorbed directly
into the cytoplasm. Indigestible particles are
removed from the cell by exocytosis



3.0 CELL DIVISION

LEARNING OUTCOMES:

At the end of this lesson, students should be able to explain:
3.1 The cell cycle
a) Show the stages of cell cycle (CLO 1)
b) Explain the stages in cell cycle: Interphase & Mitotic phase. (CLO 3)
3.2 Mitosis
a) Describe the four stages of mitosis & the behaviour of the chromosome for each stage. (CLO 3)
b) Describe briefly the cytokinesis process in animal & plant cell. (CLO 3)
c) Compare the cell division in animal & plant (CLO 1)
3.3 Meiosis
a) Define chromatid, homologous chromosome, synapsis, bivalent, tetrad, chiasma, crossing over &
centromere. (CLO 1)
b) State the stages of Meiosis I & Meiosis II (CLO 1)
c) Explain the behaviour of the chromosome at each stage. (CLO 3)

KEY TERMS

Genome All the genetic materials in a cell or individual organism includes the DNA content of the
nucleus, mitochondria & chloroplast.

Nucleotide Monomers of DNA with a deoxyribose sugar, nitrogenous bases & phosphate group.
(Nitrogenous bases consist of A,T,C & G).

DNA A double stranded, helical nucleic acid molecule consisting of a sequence of nucleotides.

Gene A discrete unit of heredity information consisting of a specific nucleotide sequence in segments
of DNA.

Chromosomes A cellular structure carrying genetic material, found in the nucleus of eukaryotic cells.
Each chromosome consists of one very long DNA molecule, coiled & wrapped around the
histone protein.

Histone protein Basic proteins that associate with DNA in the nucleus & help condense it into chromatin.
Functions: maintain the structure of the chromosome & help control the activity of the gene.

Chromatin When a cell is not dividing, each chromosome is in the form of a very long mass & thin fibers
that are not visible under light microscope.

Sister Two copies of a duplicated chromosome attached to each other by proteins at the centromere.
Chromatids

Chromatid One of the two identical halves of the sister chromatid / duplicated chromosome.

Centromere Specialized region where the two sister chromatids are most closely attached.

Kinetochore A structure of protein associated with specific sections of chromosome DNA at the centromere.
Function : attachment site for microtubules necessary to separate the sister chromatid /
chromosome during cell division.

Homologous A pair of chromosomes of the same length, centromere position & staining pattern that possess
chromosome genes for the same characters at corresponding loci.

Bivalent A pair of homologous chromosomes that line up beside each other.

Tetrad Bivalent also refers to the tetrad, synapsis of a pair of homologous chromosomes (four
chromatids).

Modern Cell All new cells are derived from other cells & enable all organisms to grow & reproduce.
Theory

Cell Division The reproduction of cells by 2 basic types which is mitosis & meiosis.

Cell Cycle The complete sequence of events in the life of an individual diploid cell.

Mitosis Division of nucleus, from one nucleus into two genetically identical nuclei.

Meiosis A type of nuclear division that results in four haploid daughter cells each having a nucleus
containing the half chromosome number (n) of the parent cell (2n).

Cytokinesis The division of the cytoplasm to form two separate daughter cells.

Centrosome A main centre in the cell that organizes the microtubules & responsible for organizing the
spindle for chromosome movements during cell division.

Gene

Chromatin

DIAGRAM ABOVE: LEVEL OF CHROMOSOME ORGANISATION

DIAGRAM: THE CHROMOSOME
CONSISTING CHROMATID, DUPLICATED

CHROMOSOME WITH SISTER
CHROMATIDS.

MODERN CELL THEORY CELL DIVISION
‘All new cells are derived from other cell’ is the reproduction of cells.
2 basic types: MITOSIS & MEIOSIS
Enable all organisms to grow & reproduce.

THE IMPORTANCE OF CELL DIVISION IN LIVING ORGANISMS

Genetic stability (mitosis) - There is no variation in genetic information. - Daughter cells are
genetically identical to the parent cell.

Cell Growth & Development (mitosis) - The number of cells within an organism increases. - This is
the basis of growth in multicellular organisms.

Cell Replacement (mitosis) - Multicellular organisms also use cell division to renew cells.

Regeneration (mitosis) - Some animals are able to regenerate the whole parts of the body; such as
tail in lizards & arms in starfish. Production of the new cells also involve cell division.

Asexual reproduction (mitosis) - Mitosis is the basis of asexual reproduction, the production of new
individuals of a species by one parent organism.

Half the chromosome number (meiosis) - Meiosis divide the chromosome number by half from the
parental chromosome. - E.g.: 46 chromosomes (parents) to 23 chromosomes (gamete).

Increased genetic variations (meiosis) - Leads the genetic variations in daughter cells due to
crossing over during prophase I & independent assortment during metaphase I.

3.1 THE CELL CYCLE

CELL CYCLE

● Definition: the complete sequence of events in the life of an individual diploid cell.

● The cell cycle consists of two main phases:
✔ Interphase (non dividing phase / the resting phase)
✔ Mitotic (M) phase (dividing phase)

● The M phase includes mitosis & cytokinesis.
✔ Mitosis - division of the nucleus.
✔ Cytokinesis - division of the cytoplasm.

1st phase of ● The longest phase in cell cycle
Cell Cycle
● It is a phase / stage before a cell can enter the mitotic phase.
INTERPHASE
● Because cell division operates in a cycle, Interphase is preceded by the previous cycle of
mitosis & cytokinesis.

● Chromosome (in the form of chromatin) invisible under light microscope.

● During Interphase, the cell grows by:

✔ Synthesis proteins & cytoplasmic organelles such as mitochondria & ER.
✔ Duplicate its chromosomes
✔ Prepares for cell division.

3 Subphases of Interphase

G1 phase (“first gap”) S phase (“synthesis”) G2 phase (“second gap”)

- Cells increase in size / cell - The longest sub-phase in - The cell continues to grow.

grows before DNA Interphase - Energy stored are increased.
replication. - Continue increases in the
- Replication / synthesis of
- Volume / mass of number of organelles such as
DNA occurs. mitochondria, Golgi body & ER.
cytoplasm is increased.
- Resulting in the DNA - Increases in the size of the
- Synthesis of protein
content of the cell being nucleus.
(histone) occurs in doubled.
preparation of DNA - Two centrosomes have formed
replication. - Chromosomes are still in
by replication of a single
- Synthesis of carbohydrate, the form of chromatin & centrosome & the cell may
become duplicated. contain a pair of centrioles in
lipid & RNA (mRNA, tRNA, the centrosome region for
rRNA & ribosomal protein) - The chromatin consists of animal cells.
occurs.
two identical DNA - The cell completes the
- Synthesis of cytoplasmic molecules (duplicated
chromosome). preparations for cell division.
organelles such as
mitochondria & ribosome - Chromosomes are - Chromatin starts to condense
that lead to the increasing
the number of organelles. duplicated during S phase, (extended & coiled) to form
and cannot be seen visible shorter & thicker
individually because they structure called chromosome.
have not yet condensed
(chromatin).

3.2 MITOSIS

2nd phase of ● Mitosis refers to division of a nucleus, from one nucleus into two genetically identical nuclei.
Cell Cycle ● Cell growth stops & ready for cell division at this stage.
● Cellular energy is focused on the orderly division into two daughter cells.
MITOTIC (M) ● Nuclear division (MITOSIS) consist of 4 phases:
PHASE
1. Prophase

2. Metaphase

3. Anaphase

4. Telophase
● Followed by division of cytoplasm called cytokinesis.

PROPHASE 1. The nuclear envelope disintegrates.
2. The nucleolus disappears.
3. Chromatin becomes shortened, thicken & more tightly

coiled / condensed into discrete chromosomes.
4. Visible under light microscope.
5. Each duplicated chromosome appears as two identical

sister chromatids joined at the centromere.
6. Each of two sister chromatids (of each chromosome has a

kinetochore, a specialized protein structure located at the
centromere.
7. The mitotic spindle / microtubules has begun to form.

8. Some of the microtubules attach to the kinetochores,
becoming kinetochore microtubules / spindle fibres.

9. Aster that extends from the centrosomes also formed.
10. The centrosomes move away from each other, apparently

propelled by the lengthening microtubules between them.
11. Centrosome - a main centre in the cell that organizes the

microtubules & responsible for organizing the spindle for
chromosome movements during cell division.

METAPHASE 1. The longest phase in mitosis.

2. The centrosome is now at opposite poles of the cell.

3. Chromosomes as a sister chromatids move & align on the
metaphase plate (equatorial plate of cell’s midline).

4. The chromosome’s centromeres lie on the metaphase

plate.

5. For each chromosome, the kinetochores of the sister

chromatids are attached to kinetochore microtubules

coming from opposite poles.

ANAPHASE 1. The centromeres split, allowing two sister chromatids to
separate / migrate.

2. Shortening of the kinetochore microtubules causes the two
sister chromatids separate & two daughter chromosome
moving towards the opposite poles.

3. Daughter chromosomes move centromere first because
the microtubule kinetochores are attached at the
centromere region.

4. The cell elongates as the non-kinetochore microtubules
from the opposite poles lengthen, overlap & push against
each other, preparing for cytokinesis.

5. Numerous mitochondria around the spindle provide energy
for movement of chromosome.

TELOPHASE 6. By the end, two ends of the cell have equivalent &
complete collections of chromosomes.

1. The chromosomes reach their respective / opposite poles
of the cell.

2. Two daughter nucleolus are formed in the cell.
3. New nuclear envelope formed around each daughter.
4. The spindle fibers disintegrate.
5. The chromosome become less condensed, which are

uncoil & lengthen, thus becoming invisible again.
6. Mitosis, the division of one nucleus into two genetically

identical nuclei, is now complete.
7. Cytokinesis (cytoplasm division to form two identical

daughter cells) begins at late telophase

DIAGRAM ABOVE: STAGES OF MITOSIS IN PLANT CELL

Cytokinesis - The division of the cytoplasm to form two separate daughter cells.
CYTOKINESIS The division of cytoplasm occurs by the late telophase, so the identical daughter cells appear

shortly after end of mitosis or meiosis II

CYTOKINESIS 1. Cleavage occurs through the
IN furrowing where the membrane
is pulled inwards by the
ANIMAL CELL cytoskeleton.

2. A shallow groove is formed in
the cell surface near the old
metaphase plate.

3. A contractile ring of actin
microfilaments forms on the
cytoplasmic side of the furrow.

4. Actin microfilaments interact
with protein myosin, causing
the ring to contract, and
reducing the diameter of the
ring.

5. The cleavage furrow deepens

until the parent cell is pinched
in two.

6. Producing two completely

separated cells, each has a
complete nucleus.

CYTOKINESIS 1. Plant cell forms a line of
IN vesicles (originating from Golgi
apparatus) inside the cell.
PLANT CELL
2. The vesicles contain materials
to construct both a primary cell
wall & middle lamella.

3. The vesicles are enlarged &
fused to form a cell plate; forms
across the midline / equatorial
of the old cell.

4. Forming two membranes which
grow laterally / expands
outward & unite / fuse with
existing membranes.

5. It thus forming two new plant
cells, each with its own plasma
membrane.

6. A new cell wall has formed
between the membranes of the
cell plate.

7. It arises from the contents of

the cell plate.

COMPARE THE CELL DIVISION IN ANIMAL & PLANT

Similarities mitosis in plant & animal cells

i. Involve four phases
ii. Both occur in somatic cells
iii. Produce identical two daughter cell

Animal cells Plant cells

● A pair of centriole present in a region called ● Only have centrosome but centriole absent.
centrosome.
● Lack of asters.
● Aster formed (the radial arrays of short
microtubules that extend from centrosome). ● Cytokinesis occurs by the growth of a cell plate through
the fusion of vesicles. It forms new cell membrane that
● Cytokinesis occurs by the formation of cleavage separate the cell into two daughter cells.
furrow. It forms shallow groove in the cell surface
near the metaphase plate. ● **Cell plate grow from inside to outside (expands
outward) to form new cell wall that separate the cell into
● **Cleavage grow from outside to inside (pulled two daughter cells.
inward) by furrowing to separate the cell into two
daughter cells. ● Occurs mainly in meristem tissues (at the root tip &
shoot apex, vascular cambium // cork cambium)
● Occurs in all somatic cells.

SIGNIFICANCE OF MITOSIS

● Genetic stability / maintain the genetic contents
✔ Mitosis produces two nuclei which have the same number of chromosomes as the parent cell.
✔ Daughter cells are genetically identical to the parent cell & no variation in genetic information can be
introduced during mitosis.
✔ This result in genetic stability within populations of cells derived from the same parental cells.

● Growth & development
✔ The number of cell within organism increases by mitosis & this is the basis of growth in multicellular
organisms.
✔ E.g.: growth / elongation / increase in body weight, organ development

● Cell replacement / regeneration
✔ Replacement / regeneration of dead / aged / damaged cells / tissue / organ involves mitosis.

● Regeneration
✔ Some animal is able to regenerate whole parts of the body; such as legs in crustacean & arms in star fish.
Production of the new cells involve mitosis.

● Asexual reproduction
✔ Mitosis is the basis of asexual reproduction, the production of new identical individuals of a species by one
parent organism. E.g.: Vegetative reproduction

3.3 MEIOSIS

Meiosis - A type of nuclear division that results four haploid daughter cell each having a nucleus containing the half
chromosome number (n) of the parent cell (2n).

● This process involved two cycle of nuclear divisions, known as
i. Meiosis I (the first meiotic division)
ii. Meiosis II (the second meiotic division)

● No DNA replication (no S phase) between the two divisions.
● DNA replication (interphase) only occurs before meiosis I.
● Meiosis is the process by which one diploid eukaryotic cell divides to generate four haploid cells often called

gametes.
● Fusion of two such cells produces a diploid (2n) zygote.
● Meiosis occurs in germ cells of (reproduction organ)

- In animals, reproduction organ is the ovary or testes.
- In plants, reproduction organ is pollen sac & ovary (or ovule).

Diploid, (2n)

refers to organism/ cell /
nucleus with 2 sets of
chromosomes, one from
mother & other one from

father.

Haploid, (n)

refers to organism / cell /
nucleus with 1 set of unpaired

chromosomes.

The main event of Meiosis I

“Separation of homologous
chromosome”

To produce 2 haploid cells

The main event of Meiosis II

“Separation of sister
chromatids”

To produce 4 haploid cells often
called gametes

MEIOSIS I

4 PHASES EXPLAINATION DIAGRAM

1. The longest phase & most complex stage in meiosis. SISTER CHROMATIDS OF A
2. All chromosomes begin to condense. CHROMOSOME ARE NO LONGER
3. The centrosomes migrated away from each other towards
GENETICALLY IDENTICAL
opposite end of the cell.
4. This phase can be divided into 5 stages:

i. Leptotene (thin threads)
ii. Zygotene (paired threads)
iii. Pachytene (thick threads)
iv. Diplotene (two threads)
v. Diakinesis (moving through)

5. The nucleoli & nuclear envelope has disintegrated.
6. The spindle fibres form.
7. 2 important events occur in Prophase I:

✔ Synapsis

- Homologous chromosome come together / close

association between the homologous chromosome results

Prophase - in pairing up of homologous chromosome.
I The resulting structure is called bivalent / tetrad.

✔ Crossing over

- An exchange of genetic material between two non-sister

chromatids of homologous chromosome at the region

called chiasmata.
- It will contribute into genetic variation / genetic

recombination.
- Chiasma - The point at which paired homologous

chromosomes remain in contact as they begin to separate

during prophase I of meiosis, forming a cross shape (X

shape).

8. In late prophase I, microtubules from one pole or the other
attach to the two kinetochores, a protein structures at the
centromeres of two homologs.

9. The homologous pairs then move toward the metaphase

plate.

Metaphase 1. The pairs of homologous chromosomes (bivalents) aligned
I on metaphase plate / equator.

2. Each pair has line up independently of other pairs.
3. The spindle, which one chromosome of each pair facing

each pole.
4. Both chromatids of the homologue are attached to

kinetochore microtubules from one pole, those of the other
homologue are attached to kinetochore microtubule from
the opposite pole.

Anaphase 1. The microtubules start to shorten.
I 2. The paired homologous chromosomes separate & pull to

opposite poles.
3. Causing the chiasmata to break.
4. This separate the chromosomes into two haploid sets, one

set at each end of the spindle.
5. Each chromosome still contains a pair of sister chromatids.
6. The centromere do not divide / split.

Telophase 1. Chromosome reach at opposite poles of the cell.
I 2. The chromosome generally decondensed into chromatin
3. The nuclear envelope may reform & surrounds each

haploid set of daughter nuclei.
4. Spindle fibers / microtubules usually disappeared
5. Due to the random orientation of homologous

chromosomes on the metaphase plate // independent
assortment, each pole receives recombination / mixture of
a set of parental chromosomes
6. Each pole of daughter nuclei now has haploid
chromosome set, but each chromosome still composed of
two chromatids
7. Cytokinesis usually occurs simultaneously with telophase I,
forming two haploid daughter cells.
8. Cell may enter a period of rest known as interkinesis.
9. No DNA replication occurs between meiosis I & meiosis II.

4 PHASES Meiosis II DIAGRAM

Prophase EXPLANATION
II
1. In each haploid daughter cells, the nucleoli and
nuclear envelopes disintegrate, and the
chromatids shorten & thicken again.

2. Centrioles, if present move to opposite poles of
the cells & at the end of prophase II, new
spindle fibers appear.

Metaphase 1. Sister chromatids align randomly at the
II metaphase plate.

2. Because of the crossing over in meiosis I, the
two sister chromatids of each chromosome are
not genetically identical.

3. The kinetochores of sister chromatids are
attached to microtubules extending from
opposite poles.

1. Kinetochore microtubules / spindle fibres begin
to contract / shorten.

Anaphase 2. The centromeres divide / split & allow the sister
II chromatids to separate.

3. The daughter chromosome moves toward
opposite poles as individual chromosome.

4. The cell elongates as the non-kinetochore /
polar microtubules lengthen, preparing for
cytokinesis

1. Nuclei form, the chromosomes begin
decondensed into chromatin.

2. Cytokinesis occurs.

Telophase 3. The meiotic division of one parent cell produces
II four daughter cells, each with haploid set of
(unduplicated) chromosomes.

4. Each of the four daughter cells is genetically
distinct from one another daughter cells and
from the parent cell.

SIGNIFICANCE OF MEIOSIS

● Meiosis ensures the constant chromosomal number is maintained from one generation to the next.
- This is done by reducing the number of sets of chromosomes from two to one in the gametes.
- Fusion of two haploid gametes in fertilization process restores the diploid condition of diploid
organisms.

● Meiosis is important in producing gametes with genetic variation/ genetic recombination by producing new
combinations of chromosomes & new combinations of alleles at different genetic loci.
- This variation arises due to the crossing over during prophase I & independent assortment during
metaphase I.

How does the genetic variation occur through meiosis?

1. In prophase 1: Synapsis /pairing up of maternal and paternal chromosomes/ homologous chromosome to form
bivalents/tetrads. During crossing over, the non-sister chromatids of homologous chromosome crossing over,
resulting in the exchange of their genetic materials at the chiasmata.

2. In metaphase I: Tetrads/ homologous chromosome aligns randomly at metaphase plate.

3. In metaphase II: Chromosome align randomly at the metaphase plate.

4. In anaphase II: Separation of sister chromatids occurs, resulting in gametes carrying the combination of
maternal and paternal genes / resulting in genetic variation.

Differences Between Meiosis & Mitosis

MITOSIS MEIOSIS
Occurs in somatic cell.
Occur in germ / oogonium & spermatogium. Megaspore
Function for repair / replacement/ mother cell & microspore mother cell / megasporocytes &
growth / asexual reproduction microsporocyte / germinal epithelial cell.
Function for gametes production

Synapsis/ pairing of homologous chromosomes does Synapsis / pairing of homologous chromosomes occur at

not occur prophase I

No chiasma occurs so there is no crossing over Chiasma occurs & crossing over between non sister
between non sister chromatids of homologous chromatids of homologous chromosome.
chromosome. Genetic variation is a result from the crossing over.

Two daughter cells produced; each are diploid (2n). Two daughter cells produced; each are diploid (2n).

The genetic composition in daughter cells is The genetic composition in daughter cells is genetically non
genetically identical to the parental cell / no genetic identical to the parent cell and to each other. contribute to
variation. genetic variation

Chromosomes align in one line in metaphase plate Homologous chromosome aligns in pairs in metaphase plate
during metaphase during metaphase I & chromosomes align in one line in
metaphase plate during metaphase II

Cytokinesis occurs once. Cytokinesis occurs twice.
Involve only one cell division Involve 2 cell divisions (Meiosis I and II)

Chromosome number of daughter cells remains the Chromosome number of daughter cells is half from the
same as the parents, 2n parents, n

Sister chromatids separate to opposite poles at Homologous chromosome separates to opposite poles at
anaphase anaphase I & sister chromatids separate to opposite poles at
anaphase II

COURSE LEARNING OUTCOMES

4.1 Mendelian genetics: monohybrid and dihybrid

a) Define terminologies used in genetic inheritance: allele, gene, locus, genotype, phenotype,
homozygous, heterozygous, dominant, recessive, self-cross and test cross. (CLO 1)
b) State the characteristics of Mendel’s pea plants. (CLO 1)
c) State Mendel’s first law (Law of Segregation). (CLO 1)
d) Construct genetic diagram on the monohybrid cross and include the genotypic ratio (1:2:1) and
phenotypic ratio (3:1) of F2 generation. (CLO 3)
e) Construct genetic diagram on Mendelian monohybrid test cross and include the genotypic ratio
(1:1) and phenotypic ratio (1:1). (CLO 3)
f) State Mendel’s second law (Law of Independent Assortment). (CLO 1)
g) Construct genetic diagram on dihybrid cross and include only phenotypic ratio (9:3:3:1) of F2
generation using Punnet square. (CLO 3)
h) Construct genetic diagram on Mendelian dihybrid test cross and include phenotypic ratio (1:1:1:1)
of F2 generation using Punnet square. (CLO 3)

4.2 Deviations from Mendelian Inheritance

a) Explain briefly types of inheritance that deviate from Mendelian: Codominant alleles, incomplete
dominant alleles, multiple alleles, linked genes, sex-linked genes and polygenes.
i. Codominant allele: Construct genetic diagram to show codominant alleles using human MN blood
group and include phenotypic ratio and genotypic ratio (1:2:1). (CLO 3)
ii. Incomplete dominant allele: Construct genetic diagram to show incomplete dominant alleles and
include phenotypic ratio (1:2:1) and genotypic ratio by using Antirrhinum sp. (snapdragon) flower
colour. (CLO 3)
iii. Multiple alleles: Construct genetic diagram to show multiple allele using human ABO blood
group. (CLO 3)
iv. Linked genes: Illustrate the effects of linked genes with crossing over on the dihybrid test cross
ratio. (CLO 3)
v. Sex-linked genes: Construct genetic diagram to show sex-linked genes using haemophilia.

(CLO3)
vi. Polygenes/Polygenic Inheritance: Explain polygenes/polygenic inheritance using human skin
colour. (CLO 3)

4.3 Genetic mapping

a) Define genetic mapping. (CLO 1)
b) Calculate the genetic distance (map unit) between genes using the given recombination
data. (CLO 3)
c) Identify the position or order of genes along a chromosome based on recombination data.
(CLO 1)

Terminologies Definition

Genetic Inheritance – Study of how traits are passed from parent to offspring.

1. Genes • A discrete unit of hereditary information
2. Alleles consisting of a specific nucleotide
3. Locus sequence in DNA (or RNA, in some
viruses).

• Units of hereditary (segment of DNA)
information about specific traits or small
section of DNA that codes for a particular
protein.

• They are passed from parents to offspring.

• Any of the alternative versions of gene that
may produce distinguishable phenotype
effects.

• Or an alternative form of a gene or trait &
occupy the same locus.

• Can be present on either one or both of a pair
of homologous chromosomes.

• Specific place along the length of a
chromosome where a given gene is
located.

4. Dominant Allele

• An allele that is fully expressed in the
phenotype of a heterozygote.

• Dominant allele is represented by capital
letter A.

• Thus if A is dominant over a, then AA and Aa
have the same phenotype.

Terminologies Definition
5. Recessive Alleles
• An allele whose phenotypic effect is not
6. Genotypes observed in a heterozygote.

• Recessive allele is represented by lowercase
of the same letter a.

• It is only expressed when homozygous, like if a
is recessive over A, only aa would show the
recessive phenotype.

• The genetic makeup, or set of allele of an
organism.

• Particular genes; individual carries / the genes
that an organism inherits from each parent.

• Genotype: TT @ Tt @ tt
(T: Tall, t: dwarf)

7. Phenotype • The observable physical & physiological
Symbol: traits of an organism, which are determined
T – dominant allele that carry the gene for Tall by its genetic makeup.
t – recessive allele that carry the gene for Dwarf
• Or an individual’s observable traits/ the protein
Phenotype : Tall Phenotype : Tall Phenotype : Dwarf used by these genes that determine the
8. Homozygous organism’s physical characteristic.

• Phenotype : Tall or dwarf.

• Same alleles at a particular gene locus on
homologous chromosomes or when both
alleles of a pair are identical.

• So, homozygous dominant has a pair of
dominant alleles (AA), homozygous
recessive has a pair of recessive alleles
(aa).

• Also known as pure breed.

9. Heterozygous

• Different alleles at a particular gene locus
on homologous chromosomes or when the
two alleles of a pair are not identical.

• So, heterozygous has a pair of non-identical

allele (Aa).

Terminologies Definition

10. Test Cross
Example 1

• A cross between a recessive

homozygous & an organism with

UNKNOWN genotype

• E.g.: Y_ X yy

• Determine the genotype of an individual

with a dominant phenotype either

individual exhibiting homozygous or

heterozygous genotype.

HINTS!!!

• If all offspring display the dominant

Example 2 phenotype, the individual in question is
11. Self Cross
homozygous dominant;

• if the offspring display both dominant

and recessive phenotypes, then the

individual is heterozygous.

• A cross between male and female
from the same plants / same
generation (F1 generation).

Characteristics of Mendel’s pea plant.
• Gregor Mendel began studied the inheritance of characteristics in garden peas (Pisum sativum)
• Pea plants have several ADVANTAGES FOR GENETICS:
• They were easy to grow.
• They had a short life cycle.
• They have easily observable characteristics.
• Their pollination could be controlled.
• In nature, pea plants typically self-pollinate.
• Mendel could also move pollen from one plant to another
to cross-pollinate plants.

• Mendel studied seven characteristics of garden pea, each of which
has two contrasting alternatives.

1. Seed shape: round / wrinkled
2. Seed colour: yellow / green
3. Pod shape: inflated / constricted
4. Pod colour: yellow / green
5. Flower colour: purple / white
6. Flower position: axial / terminal
7. Plant height: tall / dwarf

Mendel’s conclusions from these experimental results:

1. The inheritance of each trait is determined by "units" or "factors” called genes. During gamete
formation (MEIOSIS), the genes of a pair separate equally into the gametes.

2. The characters appear:
i. First generation are called dominant.
ii. Second generation are in one-quarter are called recessive.
iii. An individual inherits one such unit (gene) from each parent for each trait.

3. Recessive gene may not show up in an individual but can still be passed on to the next
generation. Each is transmitted from generation to generation as a discrete, unchanging unit.

Mendel’s First Law
Mendel’s Law of Segregation

“Two alleles for a heritable character segregate during gamete formation & end up in different gametes”.
Explanation:
Diploid cells have pairs of genes, on pairs of homologous chromosomes. The two genes of each pair are
separated from each other during meiosis, so they end up in a different gamete.

Prophase I
Homologous chromosomes

pair up to form tetrads.
Anaphase I

Homologous chromosomes
separate & are pulled
toward opposite poles.

Anaphase II
Centromeres of sister
chromatids separate

& now the separate
sisters chromatids moving

toward opposite poles.

• Mendel derived the Law of Segregation from experiments in which he followed only a single character,
such as flower colour.

• All the F1 progeny produced in his crosses of true-breeding parents were monohybrids.
Mendel’s experiments
on monohybrid cross

MONOHYBRID
• Monohybrid inheritance is the inheritance of a single characteristic.
• For this breeding experiment, Mendel chose varieties that differed in only one character.
E.g. Mendel’s breeding experiment with purple-flowered & white-flowered pea plants.
• The phenotypes ratio of dominant to recessive is 3:1 for monohybrid self-cross.
• Mendel found that the first offspring generation (F1) always has purple colour.
• However, the second generation (F2) consistently has a 3:1 ratio of purple to white.
**The recessive trait of the parent will only reappear in the F2 generation.

Construct genetic diagram on the monohybrid cross and include the genotypic ratio (1:2:1) and phenotypic
ratio (3:1) of F2 generation.

EXAMPLE 1:
In cats, brown fur is controlled by a dominant allele, B while the white fur is controlled by its recessive
allele, b. Based on the information given, answer the following questions.
Determine the expected phenotypic & genotypic ratio among F2 offspring for the cross between
homozygous dominant with homozygous recessive cats.

Answer:
B = allele controlled black fur colour
b = allele controlled white fur colour

Parental Genotype (P) :

F1 x F1 / F1 Self cross:

EXAMPLE 2:
In a certain animal, long fur (L) is dominant to short fur (l). Determine the possible offspring from crosses
between:
a) Homozygous long x short
b) Heterozygous long x heterozygous long

Answer
L = allele controlled long fur
l = allele controlled short fur

a) Parental Genotype (P) :

b) Parental Genotype (P) :

Punnett square
• Definition:
A representation of all possible genotypes produced in the F2 generation of a Mendelian genetic cross.
• Can be used for monohybrid and dihybrid.

How to create a Punnett Square for monohybrid?
• Determine the genotype of the parental generation, e.g.: Rr x Rr, or BB x bb, or YY x Yy
• Draw a square, divide it into four smaller squares.
• Above the square write the two letters corresponding the paternal genotype
• To the left, write the two letters corresponding to the maternal genotype.
Male = Bb X Female = bb
Genotypic ratio = 2 Bb: 2 bb
50% Bb: 50% bb
Phenotypic ratio = 2 black: 2 white
50% black: 50% white

Purpose of Punnett square:

• The best way to predict what types of offspring are going to be produced

Male = Aa x Female = Aa

Example

In dogs, there is a hereditary deafness caused by a recessive gene (d). A kennel owner has a male dog that

she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is
either DD or Dd. If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the

deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd).

1. Draw the Punnett squares to illustrate these two possible crosses. In each case, what

percentage/how many of the offspring would be expected to be hearing? deaf?

DD x dd Dd x dd

Genotype: All Dd Genotype: 2 Dd, 2 dd
100% can hear 50% 50%

can hear deaf

2. Also, using Punnett square(s), show how two hearing dogs could produce deaf offspring.

Dd x Dd Dd

D Dd Dd

d Dd dd

Construct genetic diagram on Mendelian monohybrid test cross and include the genotypic ratio (1:1) and
phenotypic ratio (1:1).

MONOHYBRID TEST CROSS

• Definition: A test cross can determine the unknown
genotype (heterozygous or homozygous) of an
individual with a dominant trait.

• It involves crossing the individual to a true-breeding
recessive (homozygous recessive).

• If the unknown is homozygous dominant,
all the progeny will have the dominant trait.

• If the unknown is heterozygous, approximately
half the progeny will have the dominant trait & half the
recessive trait.

• The ratio is 1:1

Example:

A= Dominant allele for tall
a = recessive allele for short

Mendel’s Second Law
Law of Independent Assortment

“Each pair alleles segregates independently of each other pair of alleles during gamete formation”.
Meiosis and the Law of Independent Assortment

Mendel identified his Second Law of Inheritance by following two characters at the same time, such as seed
colour and seed shape called dihybrids.

Construct genetic diagram on dihybrid cross and include only phenotypic ratio (9:3:3:1) of F2 generation
using Punnett square.
DIHYBRID

• Dihybrid inheritance is the inheritance of two characteristics, each controlled by a different gene at a
different locus on different chromosome.

Example 1: Green = y
Seed Colour: Yellow = Y Wrinkled = r
Seed Shape: Round = R

Phenotypic ratio:
9 Yellow, Round Seed
3 Green, Round Seed
3 Yellow, Wrinkled Seed
1 Green, Wrinkled Seed

Example 2:
In rabbits’ black coat (B) is dominant over brown (b) and straight hair (H) is dominant to curly (h). Cross a

rabbit that is homozygous dominant for both traits with a rabbit that is homozygous recessive for black coat

and straight hair. Self-cross between the F1 generation. Then give the phenotypic ratio for the first generation

of offspring.

B = allele controlled black hair H = allele controlled straight hair

b = allele controlled brown hair h = allele controlled curly hair

Phenotypic ratio: 9 black, straight hair: 3 black, curly hair: 3 brown, straight hair: 1 brown, curly hair

Construct genetic diagram on Mendelian dihybrid test cross and include phenotypic ratio (1:1:1:1) of F2
generation using Punnett square.

Dihybrid test cross
The dihybrid test cross can be carried out by crossing an individual with dominant phenotype for both
characteristics with a homozygous recessive for both characteristics individual.

Example: Y = the dominant allele for yellow seeds
R = dominant allele for round seeds y = the recessive allele for green seeds.
r = the recessive allele for wrinkled seeds

Name of Deviations Description
1. Codominant Alleles
• Definition: Both alleles in a
heterozygous organism are dominant
and fully expressed in the phenotype.

• E.g.: The existence of three different
human blood groups called the M, N and
MN blood groups.

• Another example is ABO human blood
group.

• Genotypic ratio and phenotypic ratio =
1:2:1

• Codominant allele is allele IA and allele
IB only.

2. Incomplete Dominant Allele • Definition: When neither allele
controlling a characteristic is
dominant & the aspect is displayed by
the organism results from the partial
influence of both alleles.

• Occur when dominant allele failure to
completely mask the recessive allele
in heterozygotes.

• The heterozygote first generation has
intermediate phenotype between the two
homozygous forms.

• The phenotypic ratio for the
monohybrid cross then becomes 1:2:1
in F2 generation.

Example 1
• In flower colour of snapdragons
(Antirrhinum sp.)
• A cross between a white-flowered plant &
a red-flowered plant will produce all pink
F1 offspring.
• Self-pollination of the F1 offspring
produces F2 generation with phenotypic
ratio: 1 red : 2 pink : 1 white offspring.

Name of Deviations Description
3. Multiple Alleles
Example 2

The shapes and colours in carrots are
determined by two pairs of different alleles.
Both pairs of alleles did not show dominancy.
Each genotype produces different
phenotypes. Colour of carrots are red (CRCR),
purple (CRCW) or white (CWCW). The shapes
are long (SLSL), oval (SLSN) or round (SNSN).
The long, red carrot is crossed with the round,
white carrot.
With a genetic diagram, show the parent
genotypes, gametes and F1 generation.

• Definition: One gene having more
than two alleles.

• All genes located at the single gene

4. Linked Genes locus
Name of Deviations • and control same characteristic
• In human, ABO blood group gene

consist of three alleles;
• These alleles are IA, IB and IO/i
• These alleles will determine the type

of antigen on the surface of red blood
cell.
• Allele IA and IB are codominant,
• Both are expressed in genotype in
heterozygous form.
• Allele IO/i is recessive to both alleles IA
and IB
• person with blood group A may has
IAIA or IAIO
• person with blood group B may have
IBIB or IBIO
• person with blood group AB may have
IAIB
• person with blood group O may have
ii

• Definition: Two characteristics,
each controlled by a different gene
located on same chromosome.

• Such genes do not obey Mendel’s
laws because they do not undergo
independent assortment.

• The genes are inherited together
unless separated by crossing over
during prophase 1 of meiosis.

Description

Example 1
1. A Drosophila sp. has dominant gene for
grey body (G) & normal wing (V). Recessive
allele for both genes are black body (g) &
vestigial wings (v). The fly with heterozygous
grey body & normal wing is crossed with
black body & vestigial – winged fly. The
results are:
Grey body, normal wing = 236
Grey body, vestigial wing = 53
Black body, vestigial wing = 250
Black body, normal wing = 61

a. What is the type of crossing?
b. What can you conclude about those

genes in this inheritance?

c. Show the crossing diagram.

5. Sex-Linked Genes

• Definition: Genes carried on the
sex chromosomes (X
chromosomes)

• Human have 22 pairs of
autosomes and a pair of sex
chromosome.

• Human females have two X
chromosomes, meaning they have
two sex-linked alleles.

• In males, the Y chromosome is
smaller and cannot mirror all the
genes found on the X chromosome,
so males have only one sex-linked
allele.

• This is why males suffer from the
effects of X-linked genetic diseases
more often than females.

• There are no known Y-linked traits,
probably because the Y chromosome
carries so few genes.

Sex
Chromosome

Name of Deviations Description
6. Polygenes / Polygenic Inheritance
Sex – linked traits male vs. female
• Genes for these traits are located
only on the X chromosome (NOT
on the Y chromosome).
• X-linked alleles always show up in
males whether dominant or
recessive because males have only
one X chromosome (XY).

Example: 1) Haemophilia
• Definition: The reduced ability of
blood clot, due to deficiency of
one of the blood clotting factors.
• The allele for normal blood clotting is
dominant to the allele for
haemophilia.

Example: 2) Colour Blindness
• Colour blindness is inability to
distinguish between certain
colours.
• In humans, colour vision receptors in
the retina are three different classes
of cone cells.
• One type of cone perceives blue
light, another perceives green and
the third perceives red.
• People with normal colour vision
have all three types of cone/pathway
working correctly but colour
blindness occurs when one or more
of the cone types are faulty.

• Definition: Many/ two or more
genes controlling one trait/
character.

• The traits are quantitative.
• E.g.: skin colour/ height/ weight.
• The effect of each gene alone is too

small to influence the phenotype /
All genes are needed to express
the phenotype.
• Polygenes usually exhibit a
dominant-recessive relationship.
• Only the dominant alleles contribute
to the phenotype.
• Each dominant allele contributes
equally to the overall phenotype//
each dominant allele has additive
effect.\

Name of Deviation Description

• If the genes are located on different
chromosomes, they follow the
Mendelian inheritance.

• Quantitative character can be easily
influenced by environment factors.

• Becomes the genetic basis of
continuous variation/ non discrete
variation.

• It follows the normal distribution
curve.

• The discovery of linked genes & recombination due to crossing over led to a method for constructing
a genetic map.

• This map is an ordered list of genetic loci along a particular chromosome.
• Hypothesis: the percentage of recombinant offspring (recombinant frequency) reflected the distance

between genes on a chromosome.
• The farther apart of two genes are, the higher the probability that a crossover will occur between them

& therefore the higher the recombinant frequency.
• The proportion of recombinants resulting from a dihybrid test cross is used to calculate the crossover value

(COV), a measure of linkage, and if linkage occurs, the distance between genes.
• COV is also known as recombination frequency.

Genetic Mapping Formula:
COV = total number of recombinant x 100
total number of offspring

• The lower the value, the closer the genes.
• Thus the COV can be used to locate the relative positions of genes on chromosomes, a process called

chromosome mapping or genetic mapping.
• 1% COV is equivalent to one map unit (m.u).
• Today the word centiMorgan (cM) is often used.

Example 1:

Example 2

• In tomatoes,

S = smooth s = wrinkled M = red flower m= white flower

• Test cross between smooth, red flower plant with wrinkled, white flower plant resulted in:

Smooth fruit, red flower = 300, Wrinkled fruit, white flower = 300

Smooth fruit, white flower = 100, Wrinkled fruit, red flower = 100

• Count the map unit for the distance between both S and M genes on its chromosome.

- Test cross: SM/sm x sm/sm

- Smooth fruit, red flower = 300 (SM/sm)

- Wrinkled fruit, white flower = 300 (sm/sm)

- Smooth fruit, white flower = 100 (Sm/sm)

- Wrinkled fruit, red flower = 100 (sM/sm)

Total progeny = 800

Recombinant = 100 + 100 x 100

800

= 25% S 25 M

Map unit for both S and M genes = 25 m.u

Example 3
Pure breeding sweet pea plants with purple flower (P) and long pollen grains (L) were crossed with pure
breeding plants with red flower (p) and round pollen grains (l). All F1 plants had purple flower and long pollen
grains. These F1 plants were test crossed with plants of red flower and round pollen grains and produced the
following result after the seeds produced were grown.
3998 purple flowers & long pollen grains, 403 purple flowers & round pollen grains
397 red flowers & long pollen grains, 4002 red flowers & long pollen grains

a. What can you conclude about the genes from the test cross data? Why?

Two genes involved are linked/ located on the same chromosome because the ratio from the

test cross is not 1:1:1:1

b. Calculate the map distance between the genes involved.

Map distance:

COV = total number of recombinant x 100

total number of offspring

COV = 403 + 397 x 100

3998 + 403 + 397 + 4002

= 9.09 %
= 9.09 m.u / cM
c. Draw the relative position of the genes on a chromosome.

PL

5.0 POPULATION
GENETICS

5.0 POPULATION
GENETICS

COURSE LEARNING OUTCOMES

5.1 GENE POOL CONCEPTS

a) Explain population genetics, gene pool, allele frequencies and genetic equilibrium. CLO 3

5.2 HARDY-WEINBERG LAW

a) State the Hardy-Weinberg Law. CLO 1
b) Explain the 5 assumptions of Hardy-Weinberg Law for genetic equilibrium:

i. Extremely large population size
ii. Random mating
iii. No mutation
iv. No migration / No gene flow
v. No natural selection
c) Calculate allele and genotype frequencies.

5.1 GENE POOL CONCEPT

COURSE LEARNING OUTCOMES

a) Explain population genetics, gene pool, allele frequencies and genetic equilibrium. CLO 3

1. POPULATION GENETICS

Definition: The study of genetic variability population
and of the forces that act on it. (ref: Solomon 6th ed.)
or Population genetics is the field of biology that
studies allele frequencies in populations and how they
change over time.
• Population genetics emphasizes the extensive genetic

variation within population & recognizes the
importance of quantitative characters.
• Population genetics is concerned with determining the
relative properties of the various genotypes present in
a population (genotype frequency), from which can
be calculated the relative proportions of alleles in the
population (allele frequency).

2. GENE POOL

Definition: The aggregate of all copies of every type
of allele at all loci in every individual in a population.
• The ability of a population to adapt and evolve is

thought to be influenced in part by the size of its gene
pool.
• Large gene pool indicates high genetic diversity,
increased chances of biological fitness and survival
with environmental changes.
• Smaller gene pool indicates low genetic diversity,
cause reduced chances of acquiring biological
fitness and an increased chance of extinction. (The
population may be less successful when confronted
with swift environmental change)

3. ALLELE FREQUENCIES

Definition: The proportion of a specific allele in a particular population. (ref: Solomon 6th ed.)
• In other word, allele frequency refers to how common a dominant allele or recessive allele appears in a

population.
• It is determined by counting how many times the allele appears in the population then dividing by the

total number of alleles in the gene pool.

4. GENETIC EQUILIBRIUM

Definition : Genetic equilibrium is the condition of an allele frequency and genotype frequency in a gene
pool (such as a population) does not change from generation to generation.
• The allele or genotypes frequencies is not changing across generations because the evolutionary forces

acting upon the allele are equal. As a result, the population does not evolve even after several generations.

# Detailed about genetic equilibrium is in the next subtopic, 5.2 Hardy-Weinberg Law

5.2 HARDY – WEINBERG LAW

COURSE LEARNING OUTCOMES

a) State the Hardy-Weinberg Law. CLO 1
b) Explain the 5 assumptions of Hardy-Weinberg Law for genetic equilibrium. CLO 3

HARDY – WEINBERG LAW Godfrey Hardy Wilhelm Weinberg

This law states that, in a population that is not evolving, In 1908, G. H. Hardy (an English mathematician) and W.
allele and genotype frequencies will remain constant Weinberg (a German physician) independently identified
from generation to generation. (ref: Campbell 12th ed.) a mathematical relationship between alleles and
• If a population is not evolving, it is in genetic genotypes in populations. This relationship has been
called the Hardy-Weinberg equilibrium and it concerns
equilibrium & the allele frequency do not change. allele frequency.
• When a population evolves, the allele frequency in

the population will change.
• For a population to be in genetic equilibrium, it must

satisfy five main conditions/assumptions:
i. Extremely large population size
ii. Random mating
iii. No mutation

iv. No migration / No gene flow
v. No natural selection

FIVE ASSUMPTIONS OF HARDY-WEINBERG LAW FOR GENETIC EQUILIBRIUM

If any one of these assumptions is not met, the population will not be in Hardy-Weinberg equilibrium. Instead, it may
evolve: allele frequencies may change from one generation to the next generation.

1. Extremely large population size (no genetic drift) Figure 5.2.1: The population is not in Hardy-Weinberg equilibrium
• Genetic drift involves changes in allele frequency due to genetic drift that have occurred in the small population.
due to chance events – literally, "sampling error"
in selecting alleles for the next generation.
• Genetic drift can occur in any population size, but
it has a stronger effect on small populations size.
• In extremely large population size, the genetic
drift can be avoided, because minor change in
the frequency of alleles is not significant.
• But in small populations size, genetic drift can
cause genotype frequencies to change over
time. (see figure 5.2.1)

2. Random mating (no sexual selection)
• Each individual in a population has an equal
chance of mating with any individual of the
opposite sex in order to allow the random
mixing of gametes. (see figure 5.2.2)
• If individuals’ mate within a subset of the
population, such as near neighbours or close
relatives (inbreeding), random mixing of
gametes does not occur and genotype
frequencies change.

Figure 5.2.2: Random mating allow random mixing of gametes.

3. No mutation (no genetic change)

• Mutations occur by changing one allele into
another, or if entire genes are deleted or
duplicated, in other word, mutation alter the
gene pool.

• Mutation that occurs during cell division can
create a new type of gene. − That new gene is
a small part of the gene pool that can be
passed on to the next generation. thus, the
gene pool will be altered. (see figure 5.2.3)

4. No migration / no gene flow in or out Figure 5.2.3: The population is not in Hardy-Weinberg
• Gene flow involves the movement of genes into equilibrium due to mutation
or out of a population, due to either the
movement of individual organisms or their Figure 5.2.5: The frequencies of allele changed in the next
gametes (eggs and sperm, e.g. : through pollen generation due to natural selection
dispersal by a plant).
• Organisms and gametes that enter a
population may have new alleles, or may bring
in existing alleles but in different proportions
than those already in the population.
• By moving alleles into or out of populations,
gene flow can alter allele frequencies.

5. No natural selection
• Natural selection occurs when one allele (or
combination of alleles of different genes)
makes an organism more or less fit, that is,
able to survive and reproduce in a given
environment.
• If an allele reduces fitness, its frequency will
tend to drop from one generation to the next.
(see figure 5.2.5)
• Allele frequencies change when individuals
with different genotypes show consistent
differences in their survival or reproductive
success.

COURSE LEARNING OUTCOMES

c) Calculate allele and genotype frequencies. CLO 3

Allele frequencies

HARDY- p+q=1 Why p + q equal to 1?
WEINBERG
EQUATION p : frequency of dominant allele Because there are only two possible
q : frequency of recessive allele alleles, we can say that the frequency
The Hardy-Weinberg of p and q together represent 100%
equation is a Genotype frequencies of the alleles in the population.
mathematical
• With the allele frequencies of a population, we can use an extension to
equation that can be calculate the expected frequency of each genotype following random mating
used to calculate within the entire population.

the genetic variation • This is the basis of the Hardy-Weinberg formula:
of a population at
equilibrium. p2 + 2pq + q2 = 1

p2 : frequency of homozygous dominant genotype
2pq : frequency of heterozygous genotype
q2 : frequency of homozygous recessive genotype

Phenotype frequencies

Frequency of dominant phenotypes : p2 + 2pq

Frequency of recessive phenotypes : q2

Final answer for frequency must be written in decimal point. The
number of decimal points depend on the place value of the sample.

Place value of the sample Number of decimal point
Ones
Tens 1 decimal point
Hundreds
Thousands 2 decimal points
Ten thousands 3 decimal points
Hundred thousands 4 decimal points
5 decimal points